1. Field of the Invention
This invention relates to test probes for non-destructive evaluation and testing and particularly to a new and improved ultrasonic liquid jet probe which is configured for discharging a stable, uniform liquid jet column and for coupling unmodified ultrasonic waves with a structure.
2. Description of the Prior Art
Ultrasonic waves are often employed in the non-destructive testing of materials and structures, particularly to locate inhomogeneities and defects within the structures or to assess structural integrity and material properties. In one presently utilized method of nondestructive testing, a transducer emits ultrasonic waves which are coupled with the structure being tested by a liquid jet column discharged from a probe. In a particular arrangement, known as the pulse echo mode, the ultrasonic waves are reflected back to the same transducer via the liquid jet column. In a more common arrangement, known as the through transmission mode, the waves pass through the structure and are carried to a receiving transducer by another liquid jet column. In either case, the transducer then converts the waves into electrical signals which are appropriately processed.
Attenuation and spurious reflections of the ultrasonic wave signal, however, reduces the accuracy and reliability of any such liquid jet probe system, but particularly of systems based on the pulse echo mode, that mode being advantageous for reasons of greater compactness and simplicity and because it does not require access to both sides of the test material. Causes of the ultrasonic wave signal degradation include instability and gravitational droop of the liquid jet column and wave reflections within the probe housing.
Instability of the liquid jet column results in a corrugated, asymetric column surface or turbulence within the column causing not only reflections which tend to mask the true ultrasonic waves, but also a variable and unpredictable amplitude and wave-form of the transmitted ultrasonic signal.
Droop of a nonvertically-oriented liquid jet column occurs when the end of the column curves downwardly due to gravity. The downward displacement of the column is inversely proportional to the flow velocity of the liquid in the column. To avoid signal degradation, it is required that the waves travel essentially undeflected in a straight line and often normally incident to the surface of the test structure. Therefore, in many prior art arrangements in which the liquid jet column is discharged at low flow velocities, the probe must be spaced close to the test surface so as to avoid droop. Although increasing the velocity of the liquid jet column would decrease droop, the higher velocity tends to also create flow irregularities within the column which, as was described above, interferes with the ultrasonic wave signal. It may also be desirable to induce relative motion between the test piece and the probe in order to evaluate large areas of the test piece. If, in available systems which require that the probe be spaced closely to the test piece to avoid droop, the surface of the test piece is of a complex geometric form, elaborate arrangements may be required to enable the probe to follow the surface contour and to avoid ultrasonic signal degradation due to probe orientation.
In many existing probes, the configuration of internal flow passages results in flow instabilities, eddies, unsteadiness and turbulence. These undesirable flow disturbances and irregularities, as well as the materials from which existing probes are fabricated, lead to improper shaping of the ultrasonic wave-form and internal wave reflections as the ultrasonic waves pass through the probe. Such reflections have inhibited attempts to develop workable systems operating in the pulse echo mode where the internal reflections interfere with the true wave reflections from the structure being tested.
In view of the above mentioned problems, it is therefore an object of the present invention to improve the wave coupling performance of ultrasonic liquid jet probes by configuring the probe to discharge a uniform, stable liquid jet column.
Another object of the present invention is to permit an increase in the distance between the probe and the test structure by decreasing the gravitational droop of the liquid jet column.
Another object of the present invention is to provide a probe in which ultrasonic performance is independent of probe orientation.
Yet another object of the present invention is to properly guide ultrasonic waves to the water jet column and to reduce ultrasonic wave interference by reducing internal wave reflections within the probe housing.
Still another object of the present invention is to provide an ultrasonic liquid jet probe of such improved sensitivity and accuracy and low amplitude noise that it can be used in either a through transmission or a pulse echo mode.